Broadcast color video signals conventionally comprise three component signals: a luminance signal, designated as Y, which carries the image contrast--black and white--information, and two chrominance signals, designated either as I and Q or (R-Y) and (B-Y), which carry the image color information. Composite video, such as the NTSC video signal that is the standard for color television transmissions in North America, combines the three component signals for transmission in analog form by modulating a carrier with both of the chrominance signals in phase-quadrature and then interleaving the combined chrominance signal with the baseband luminance signal. Component video, however, treats the analog baseband luminance and chrominance signals as separate channels and does not combine them for transmission in analog form. Component video produces a higher-quality image than composite video because it avoids crosstalk between the three components that would be introduced by combining them in composite video, and because it can allow for more bandwidth for the chrominance components of the signal than does the NTSC composite video format.
Because digital signal transmissions are less susceptible than analog signal transmissions to noise and other image degradations introduced during transmission, the quality of a received video image can be enhanced further by means of digital video signal distribution. The frequency, or bit rate, required for digital transmission of component video is determined by the sampling rate and bits per sample for each of the luminance and chrominance signals. The sampling rate and the number of bits used per sample are generally directly related to the image resolution and quality. Image quality is thus generally directly related to the transmission bit rate, and hence it is desirable to keep the transmission rate at a maximum. However, conventional transmission facilities are generally limited in the transmission rate that they can handle, and the cost of a transmission facility is generally directly related to the maximum rate that it can handle. Hence it is desirable to keep the transmission rate to a minimum.
These conflicting considerations have resulted in numerous techniques and arrangements that have imaginatively sought to reduce the transmission rate without sacrificing greatly the video image quality in the process. An example of such a technique is to greatly limit the bandwidth of chrominance signals--to 0.5 MHz, for example--to reduce the frequency at which the signals need be sampled. This technique, however, leads to excessive distortion of, and lack of resolution in, the image color. Information on color detail is carried at the higher frequencies. Hence the lower is the chrominance bandwidth, the more of the color detail is lost from the image.
Another example is a technique that takes advantage of the periodicity of the video signals' spectrum to reduce the sampling rate, by sampling both the luminance and chrominance signals at sub-Nyquist frequencies. The Nyquist frequency is twice the maximum frequency contained by the signal and theoretically is the lowest frequency at which a given signal may be sampled such that the samples retain all information content of the sampled signal. This technique then uses comb filters to remove aliasing--distortion--spectral components that are introduced into the baseband video signals by sub-Nyquist sampling. While sub-Nyquist sampling is an attractive approach, it, like any compression technique, produces image degradation. For sub-Nyquist sampling, the degradation comes in the form of distortion of diagonal patterns. The amount of distortion of diagonal patterns is directly related to how deeply the combing must extend into the baseband of the video signals to remove the aliasing spectrum. And if the combing is done throughout the video baseband, further degradation comes in the form of reduction in the vertical resolution of the image.
Yet another technique uses expensive band-limiting filters that have a very sharp cutoff of their pass-band region, to minimize the spectral content of the video signals, and then uses as low a super-Nyquist sampling frequency as possible. With this approach, the filter design and realization becomes very complex and expensive. In fact, the design becomes a compromise between cost and complexity of the filters versus the amount of degradation of the image that results from non-ideal filter characteristics. Most of the filters start out with a minimum phase filter design that will meet the selectivity, i.e., steepness of cutoff, requirements imposed by the choice of sampling frequency. But the phase distortion caused by this type of filters is proportional to the selectivity of these filters. Since the required selectivity is high, there is an associated high phase non-linearity, especially in the higher frequency region of the baseband video signals. The phase non-linearity causes a differential delay of the high frequencies, which produces "ringing". Ringing is a very noticeable picture degradation. Hence, the phase distortion introduced by such filters must be minimized, by the addition of phase shift equalization circuitry. However, phase shift equalization typically is difficult to perfect for very sharp cut-off filters, and the circuits needed to realize it are typically costly and complex.
The techniques that were just discussed may have been adequate for transmissions of conventional color television images, in spite of the fact that they result in some amount of image degradation. However, none of these techniques alone are satisfactory for transmissions, at rates compatible with conventional transmission media, of high-quality component video images or images such as those that are required for High Definition Television (HDTV). The degradation of image quality produced by known arrangements using these techniques is just too severe, and the cost and complexity of certain of their component parts is too high.